1. Field of the Invention
The subject of the invention is a process for making photographic recording materials for medical radiography. Whether used with green-emitting intensifying screens or blue-emitting intensifying screens, these materials are characterized by comparably good photographic and physical properties.
2. Description of Related Art
Medical radiography uses photographic recording materials having at least one radiation-sensitive silver halide emulsion coating on both sides of a support (hereinafter designated as x-ray films) in combination with intensifying screens. Intensifying screens contain calcium tungstate or rare earth phosphors, and depending on their composition, emit light of certain wavelengths when irradiated with x-rays. Therefore, the x-ray films must be sensitive in the range of these wavelengths.
Rare earth intensifying screens are known to emit essentially either green, blue or ultraviolet light when irradiated with x-rays. The intensification by the screen and its effect on the visual resolution and noise in the resulting x-ray images depend on the composition of the intensifying screens. The intensifying screen's contributions to the system sensitivity and to the visual resolution in the corresponding x-ray film/intensifying screen system are related inversely. A classification of intensifying screen sensitivities and relative film sensitivities is given in "Image Quality in X-ray Diagnostics" ("Bildqualitat in der Roentgendiagnostik"), by H. S. Stender and F. E. Stieve, Deutscher Aerzte-Verlag Koeln, 1990, pages 168-175. Spectral sensitivity S of a film/screen combination is determined by the exposure K.sub.s, measured in mGy, required to achieve a film density of 1.0 above fog: EQU S=K.sub.0 /K.sub.s with K.sub.0 =1 mGy.
Based on the sensitivity thus obtained, the film/screen combination is conventionally assigned to a sensitivity class. Common sensitivity classes are 50, 100, 200, 400, and 800. A certain film/screen combination is assigned to the class wherein the sensitivity is closest to the class sensitivity. For example, any film/screen combination with a sensitivity of 301 to 600 would be assigned to the sensitivity class 400.
Commercial x-ray films are classified in accordance to their relative speed when exposed to blue or red light, respectively, as half, standard, or double speed films or to relative film speed classes of 0.5, 1.0, or 2.0.
Commercial intensifying screens are also assigned to speed classes such as 50, 100, 200, 400, and 800, in accordance with their contribution to system sensitivity S.
The system sensitivity class of a given film/screen combination can be obtained as the product of the screen speed class number and the relative film speed given in the tables of the Stender et al. publication. For example, the sensitivity class of the system consisting of DuPont Quanta Fast Detail screen and Cronex.RTM. 7L is 400*0.5=200.
The physical and photographic properties of the x-ray films together with the composition of the intensifying screens determine the quality of the resulting x-ray images and, thereby, their suitability for accurate medical diagnosis by radiology. The highest possible visual resolution in the x-ray image at the lowest possible patient exposure to radiation is especially desirable. Accordingly, the combination of x-ray film and screens must by very accurately coordinated. The various diagnostic problems for the radiologist usually involve, in practice, the use of different combinations of x-ray film with intensifying screens of various sensitivities and, consequently, various system sensitivities.
The silver halide emulsion for making x-ray films must have the highest possible sensitivity. The expert knows, however, that using the largest possible silver halide grains to attain higher sensitivity has limits, because the quality (resolution and noise) of the x-ray images decreases with increasing size of the silver halide grains.
Many methods are known to improve sensitivity or the ratio of sensitivity to fog in silver halide emulsions. For example, a conventional method is chemical sensitization by chemically ripening with gold and with sulfur. Another method is the so-called reduction sensitization. In addition, other materials are known to affect the sensitometry of silver halide emulsions, for example, 2-amino-ethanethiol-1-hydrochloride or 3-thiazolidine-4-carboxylic acid, as described, for example, in U.S. Pat. No. 2,860,976 and DE-PS 17 72 424.
The sensitivity and gradation of an x-ray film can be determined, for example, according to German Standard DIN 6867 (April 1985) or as described in "Image Quality in X-Ray Diagnostics", by H. S. Stender and F. E. Stieve, Deutscher Aerzte-Verlag Koeln, 1990.
Suitable green-sensitizing dyes and their use in photographic recording materials are known to the expert. Examples are trimethinecyanine dyes, optionally substituted appropriately. Such dyes are described, for example, in EP-A 0 581 065 and EP-A 0 404 142. A general review of the use of chemical and spectral sensitization of photographic silver halide emulsions is presented in Research Disclosure, Vol. 308, Number 308119 (December 1989), Chapter III and IV and the references therein.
Known green-sensitized silver halide recording materials have a significantly higher sensitivity to the light produced by green-emitting intensifying screens than to the light produced by blue-emitting screens. Examples are described in EP-A 0 264 788 and EP-A 0 581 065.
Known blue-sensitive and green-sensitive x-ray films and the corresponding intensifying screens are listed, for example, in "Image Quality in X-Ray Diagnostics", by H. S. Stender and F. E. Stieve, Deutscher Aerzte-Verlag Koeln, 1990, pages 168-174.
The current state of the art has a disadvantage for the radiologist, because in the use of an x-ray film with a certain spectral sensitivity, the selection of a suitable intensifying screen is limited to a group of intensifying screens with the corresponding emission spectrum.
In addition to the invariably high quality requirements on modern x-ray films, rapid availability of the developed x-ray image is also important. Examples are images that are made during an operation and that are supposed to indicate the further course of the operation. In addition, the images must be rapidly available so that the radiologist sees whether the diagnostically significant area has been imaged, while the patient is still under the x-ray camera.
Furthermore, exposed material from many imaging devices, such as, for example, x-ray cameras, laser cameras, apparatus for photographic monitoring, and copy machines for x-ray films are often processed in the same processor in hospitals and large medical practices. Therefore, the highest possible throughput of photographic films and, consequently, the shortest possible processing time of less than 60 seconds are desired in the processor for x-ray films and for other photographic films in those hospitals and medical practices.
The processing time for a photographic film depends decisively on the composition of the film, the structure and mode of operation of the processor, the developer solution used in the processor, and the fixing bath. All parameters, such as, for example, dryer geometry, drying time in the processor, or process water uptake by the film, which affect drying the film in the processor, are especially important.
Processing time is defined as the time that a standard 0.35 m.times.0.35 m edge length x-ray film requires to pass through the processor, beginning with the insertion of the x-ray film and ending with the complete exit of the developed x-ray image. This time span is designated in the literature as "nose to drop".
A photographic silver halide recording material qualifies as rapidly processible, if it can be processed within 30 to 60 seconds in a film processor. A necessary prerequisite for the recording material is a maximum process water uptake of 20 g/m.sup.2.
An example of a roll film developing machine suitable for this processing and a formulation for a developer and fixer bath for the machine are described in EP-A 02 38 271.